1. Field of the Invention
This invention relates to a color image forming apparatus such as an electrophotographic type color copying machine or a color laser printer.
2. Description of Related Art
Numerous image forming apparatuses using an intermediate transferring body have heretofore been proposed as color image forming apparatuses making the most of a feature such as the capability of coping with various transferring materials (recording media), and particularly superimposing a plurality of colors one upon another.
FIG. 9 of the accompanying drawings shows an example of a color image forming apparatus using an intermediate transferring belt. In FIG. 9, on the peripheral surface of a photosensitive drum 101 as a first image bearing member, there are disposed charging means 102, various color developing means of 106 (black), 107 (magenta), 108 (cyan) and 109 (yellow), an intermediate transferring belt 110 as a second image bearing member, and a photosensitive drum cleaner 118, and the color developing means 106–109 are adapted to contact with the photosensitive drum 101 by means (not shown) as required. The intermediate transferring belt 110 is passed over a drive roller 115, a secondary transfer opposing roller 116 and a tension roller 117, and is rotatively driven-in the direction of the associated arrow by the drive roller 115.
The photosensitive drum 101 is rotatively driven in the direction of the associated arrow, and is uniformly charged by the charging means 102 to which a bias of the negative polarity is applied from a bias voltage source 103, and a laser beam 105 comprising a modulated signal is applied thereto by exposure means 104 which is information writing means, whereby an electrostatic latent image is formed on the photosensitive drum. Next, a toner which is a developer charged to the same polarity as the above-mentioned charges is supplied onto the photosensitive drum 101 on which the electrostatic latent image has been formed, by the developing means 106–109, respectively, whereby the electrostatic latent image portion is made into a visualized toner image. Thereafter, a voltage opposite in polarity to the toner is applied to a primary transferring roller 111, which is first transferring means, by a primary transferring bias voltage source 112, and the toner image is electrostatically transferred to the intermediate transferring belt 110.
In the case of plural-color image print (full-color print), the above-described step is repeated four times in all (four-path system) by the developing means 106–109 for a plurality of colors to thereby form a color image on the intermediate transferring belt 110. Thereafter, a voltage opposite in polarity to the toners is applied to a secondary transferring roller 113 which is second transferring means by a secondary transferring bias voltage source 114 through a transferring material 120 to thereby collectively transfer the color image onto the transferring material 120 such as paper, and a color image print is obtained as a permanent image by a fixing apparatus 121.
Also, any primary untransferred toners on the photosensitive drum 101 after the primary transferring step are collected by a cleaner 118, and any secondary untransferred toners on the intermediate transferring belt 110 after the secondary transferring step are collected by a cleaner 119. The cleaner 119 is rockable in the direction of the associated arrow, and is controlled so as to be spaced apart from the intermediate transferring belt 110 when each color toner image is being primary-transferred to the intermediate transferring belt 110, and to abut against the intermediate transferring belt 110 after a four-color toner image has been formed on the intermediate transferring belt 110.
Also, in the case of single-color image print (monocolor print or monochrome print mode), a black toner image by the operation of the black developing means 106 is formed on the photosensitive drum 101, and it is primary-transferred to the intermediate transferring belt 110, and then is secondary-transferred onto the transferring material 120 in a secondary transferring part, and a monocolor print is obtained as a permanent image by the fixing apparatus 121.
The fixing apparatus 121 adopts the construction of a film fixing apparatus using as a heating member cylindrical fixing film heated by a ceramic heater or the like provided therein, and the heating member is temperature-controlled on the basis of a temperature detected by a thermistor which is temperature detecting means so that the surface temperature of the fixing film may become a control target temperature by a heater driving circuit. At this time, the control target temperature changes to a plurality of stages in conformity with the number of printed sheets per job (see e.g. Japanese Patent Application Laid-Open No. H5-165368).
However, when the aforedescribed conventional fixing apparatus is used in a color image forming apparatus, there has been the problem that the toner image on the transferring material is offset during plural-color image print. This will hereinafter be described in detail.
The above-noted phenomenon is the phenomenon that when temperature is controlled by the control target temperature during single-color image print, in the case of plural-color image print, the temperature of a pressure roller rises and the toner on the transferring material is excessively melted and some of the toner on the transferring material is offset to the fixing film side.
According to the applicant's studies, it has been found that during plural-color image print, as compared with during single-color image print, the paper transfer interval becomes wide and therefore during continuous print, the temperature of the pressure roller becomes higher than during single-color image print. That is, during plural-color image print, the temperature of the fixing film is substantially the same as that during single-color image print, but the temperature of the pressure roller is high and therefore, the temperature of the entire fixing apparatus becomes high, and the amount of heat applied to the toners on the transferring material increases. Therefore, the toners on the transferring material are melted too much, and when the transferring material passes through a nip part which is the portion of contact between the fixing film and the pressure roller, a part of the too much melted toners is offset onto the fixing film. These offset toners are fixed on the transferring material after the fixing film has made a round, thereby causing a faulty image.